Bass traps - foam cushiony items from Amazon ok?

I tried building a variable helmholtz resonator once.  It didn't work out too well and didn't really do anything.  I found that tuned membrane bass traps worked a lot better.

Foam is really not a good acoustic treatment.  Sure, it's cheap, but I have had weird results with foam.  The best acoustic panels are made from fiberglass strands or similar.  The cheapest of these are from GIK (or you can actually make them yourself from something like Owens 703 panels (just google for ATS Acoustics). 

For bass traps, you really need a very thick panel (like 12-14" or more).  Either that or tuned membrane bass traps such as the Scopus series from GIK Acoustics.  2-4" panels are not effective at bass frequencies.